This invention relates generally to the production of titanium slag from ilmenite and is more particularly concerned with reducing the radioactive content in the slag product.
The removal of radionuclides from titanium bearing materials is of importance in the manufacture of feedstock for the titanium pigment industry. Titanium oxide pigments are produced from various titanium bearing materials such as rutile, ilmenite, titanium slag and synthetic rutile. The presence of radionuclides is common but is not acceptable to the pigment manufacturers because the radionuclides accumulate in the residue of manufacture and constitute a difficult disposal problem.
Various processes are used for upgrading titanium bearing pigment feedstock. These usually are based on the use of some means for removal of impurities such as iron, manganese, alkali elements and other minor elements. Some of these processes are effective in the removal of radionuclides even though the processes are not specifically designed for radionuclide removal. On the other hand in cases where the titanium bearing material is already substantially free of impurities the cost of these processes is an unnecessary expense. There is a need for a process which will not add substantial cost to the existing processes for converting titanium bearing materials to pigment feedstock.
Common processes involve the conversion of ilmenite to synthetic rutile and the conversion of ilmenite to titanium slag. These processes remove mainly iron but also small amounts of other elements. These processes do not however have the effect of removing radionuclides unless certain additional effort is applied for this purpose.
By way of example the specifications of South African patents Nos. 93/5922 and 93/5474 describe processes to upgrade titaniferous materials by removing impurities. These processes are designed to be compatible with the production of synthetic rutile and involve the removal of radionuclides. The processes can be applied to titanium bearing materials such as ilmenite or to the product of any process used to produce synthetic rutile, and can also be used as a replacement for some of the steps in the production of synthetic rutile.
The aforementioned processes provide for the addition of compounds which form a glassy phase during a heat treatment step. Impurities, particularly radioactive elements, migrate to the glassy phase. Subsequent leaching steps are effective in dissolving the radioactive elements out of the glassy phase. A typical additive to enhance the solubility of the glassy phase is borax but other components containing boron and oxygen may also be used. The heat treatment process together with the addition of borax can be conducted on ilmenite before conversion to synthetic rutile or titanium slag.
Under some conditions it is attractive to convert ilmenite to titanium slag. This has been done in some installations which make use of electric furnaces where coal or another reductant is used to convert ilmenite by reduction into iron metal and titanium slag. DC arc furnaces have also been used in this process. Any radioactivity present in the ilmenite will appear in the slag after reduction in the electric furnace.
The specification of South African patent No. 95/3046 teaches that borax can be added to titanium slag before roasting the mixture at about 1000.degree. C. The cooled slag is leached with alkali and acid and the silica content of the slag is reduced. A reduction in radioactivity is not mentioned but it is expected that this would occur because similar processes applied to other titanium bearing materials are effective.
If however this technique is used for radioactivity removal an additional roasting and leaching step must be applied either to the ilmenite before slag production or to the slag after its production.
If it is desired to produce titanium slag then it is preferable to produce a slag product which is low in radioactivity without the additional roasting process.
The specification of U.S. Pat. No. 3,829,309 teaches that smelting of ilmenite to produce a borate-titanate slag can be conducted at 1200.degree. C., which is a much lower temperature than that used in an alternative known process of smelting ilmenite without fluxing. This process has been carried out in a rectangular AC furnace with six electrodes in line, although a DC arc furnace can be used in place of the AC furnace. The smelting action takes place at 1700.degree. C. and molten pig iron and titanium slag are tapped into ladles.
The aforementioned American patent discloses that the slag produced is leached with water to remove borate and the resulting slag is also leached with water to produce a feedstock to the pigment industry.
The process, which uses borax as a flux, has the following disadvantages:
(a) the slag is not of a very high grade--typically 70% to 80% TiO.sub.2. If the grade must be higher then the recovery rate of titanium from the slag phase is low. PA1 (b) the cost of the process is high. Firstly losses of borate are high and secondly, as taught by the patent, air blowing of the slag is necessary to oxidise the titanium. PA1 (a) smelting ilmenite in an electric arc furnace in the presence of a reductant to produce a slag, PA1 (b) adding a boron compound in an amount of from 0.5% to 2.5% equivalent B.sub.2 O.sub.3 to the slag, PA1 (c) cooling titanium slag which is tapped from the furnace, PA1 (d) crushing the cooled slag, and PA1 (e) leaching the crushed slag to dissolve radioactive elements in the slag into the leaching material. PA1 (I) prior to step (a) by adding the boron compound to the ilmenite; or PA1 (ii) during step (a) by introducing the boron compound into the furnace; or PA1 (iii) after step (a) by adding the boron compound to titanium slag while the slag is molten.